Recent advancements in computer technology have evolved the conventional desktop personal computer into portable versions. Essentially, the only limiting factor in the progressing miniaturization of the computer is the physical characteristics of the human body. A user still requires a keyboard to enter data into the computer and a display screen to observe computer data. Keyboards must be at least a minimum size for the human hand to effectively operate the keys, and the human eye is strained to observe images below a minimum size. Thus, the latest models of the portable or lap-top computers still require an adequate display so that a user may effectively use the computer.
Desktop computers generally take advantage of the full capabilities of cathode ray tube ("CRT") displays. Their bulky size is not restrictive since the desk-top computer is not required to be mobile. However, bulky CRT displays are not at all practical for portable or lap-top computers. As a consequence, flat panel displays have been developed for portable computers so that they may remain small and lightweight. Such flat panel displays are most typically of the liquid crystal display ("LCD") type. The disadvantage of such displays is that they are difficult to view in bright rooms or sunlight since they do not generate their own light as do CRT displays. The solution to this problem is found in so called "backlit" displays whereby an external light source is utilized to illuminate the flat panel display. Such light sources may be, for example, cold cathode fluorescent lamps disposed on the sides of the display screen. The lamps are generally powered by a voltage supply and allow a user to adjust either or both the contrast and brightness of the display.
Since one of the advantages of a portable computer is its mobility, it needs to be able to operate on a DC power supply such as a rechargeable battery located within the computer chassis. Consequently, the backlit lamps must also receive their power from the DC source. However, such lamps generally require an AC power source which is not available when the computer is powered by its internal DC battery. Therefore, an inverter circuit is required to transform the power source from DC to AC for utilization by the lamp.
The addition of the fluorescent lamp and its associated inverter circuit introduces the possibility of yet an additional problem. By the very nature of a portable computer, its contents must be able to withstand greater physical stress resulting from the repeated transportation of the computer. This is not as great a concern with respect to inverter circuits as it is to fluorescent lamps, as they tend to be more susceptible to damage.
An additional concern is the proper installation and connection of the fluorescent lamp during manufacture. Even with tight quality controls, a computer may occasionally depart the manufacturing facility with an improperly connected fluorescent lamp.
A simple solution to the aforementioned problems is to either replace the lamp if it is damaged or properly connect it to the inverter circuit if that remains to be done. Such solutions, though bothersome to the owner of the computer, are relatively simple and inexpensive. The greater concern is the possibility of damage to the components of the inverter circuit if the fluorescent lamp is not properly connected to the circuit or if the lamp breaks during operation. Upon such occurrences, an open circuit results between the connections of the fluorescent lamp at the output of the inverter resulting in its high voltage transformer and driver components (field effect transistors and diodes) becoming overly stressed and eventually failing. It is possible to design a transformer that can withstand a surge in voltage and not burn up, but such a transformer is substantially more expensive and physically larger and heavier.
A not uncommon occurrence is for the lamp to break due to a mishandling of the computer. Thereafter, when the user turns on the computer, it goes through its normal power up routines and attempts to turn on the lamp through the inverter circuit. Because the output of the circuit is not "closed" by the presence of a working lamp, the voltage in the transformer increases up to about 5,000 to 15,000 volts attempting to illuminate a faulty load. Often, other components within the inverter circuit will fail. Unfortunately for the user, a transformer or complete inverter circuit is much more difficult and expensive to replace than a fluorescent lamp.
Therefore, what is needed is an inverter protection circuit for protecting the transformer and the remainder of the inverter circuit upon the occurrence of a fault, such as when the fluorescent lamp is improperly connected to the inverter circuit or when it has been broken during use as described above. Such a protection circuit would also be useful and applicable to any piece of equipment where high voltage AC is derived from a DC source, such as in calculators, liquid crystal television displays and electroluminescent displays.
There have been several attempts in the past for providing such a protection circuit. FIG. 4 shows one prior art circuit for protecting a cold cathode fluorescent lamp. However, no provision has been made within this circuit for when the lamp is either broken or not properly connected resulting in an open circuit at the location of the lamp. Thus, this circuit cannot protect against all possible lamp faults.
FIG. 5 shows another prior art protection circuit wherein a thermal fuse is the means of protection. This fuse is in thermal contact with the high voltage transformer. When the transformer heats up because of increasing voltage, the thermal fuse will eventually blow, shutting down the entire circuit. However, during the interval before the fuse blows, the transformer and other important components of the inverter circuit are still voltage--stressed. Thus, this circuit does not have sufficient response time to ensure minimal inverter circuit stress.
FIG. 6 shows yet another prior art protection circuit that does provide for protection of the circuit in the instance where there is "no lamp." When the lamp circuit is open, the voltage at transistor Q9's collector is higher than diode D3's breakdown voltage, resulting in current flowing into a node of resistor R24, capacitors C12 and C2 and resistor R22. This causes inverter output to be reduced to safe limits, but still does not result in a total shutdown of the inverter circuit, which is the best means of protecting the transformer and its associated components.
What is needed is an effective protection circuit for an inverter circuit supplying power to a fluorescent lamp. In a preferred embodiment, the fluorescent lamp is a cold cathode fluorescent lamp ("CCFL"). The protection circuit should quickly turn off the power to the inverter circuit and its transformer upon the occurrence of a disconnected or broken CCFL, or when the CCFL fails to light for some other circuit malfunction, so that the high voltage transformer and driver components are not overly stressed and fail.